A piston has hitherto been proposed, which comprises an outer piston (moving piston unit) and an inner piston (base piston unit). The inner piston is a component not integral with the outer piston, and has a piston boss to which a connecting rod may be coupled. The outer piston is able to reciprocate with respect to the inner piston. The piston of this type has a rotation preventing structure that prevents the outer piston from rotating with respect to the inner piston.
More specifically, the outer piston has a circumferential wall part all surrounding the inner piston. A hole is made in the circumferential wall. Through the hole and the piston boss, a rotation preventing member extends, preventing the outer piston from rotating with respect to the inner piston. The wall appears like a hollow cylinder. Such a technique is disclosed in, for example, JP H06-17665A.